Sensor system for lithography

ABSTRACT

A sensor system to measure a physical quantity, the system including a parallel detection arrangement with multiple detectors to allow measurements in parallel at different spatial locations, wherein the multiple detectors share a noise source, wherein the sensor system is configured such that the multiple detectors each output a signal as a function of the physical quantity, and wherein the sensor system is configured such that at least one detector responds differently to noise originating from the shared noise source than the one or more other detectors.

This application a continuation of U.S. patent application Ser. No.14/436,046, which was filed on Apr. 15, 2015, now allowed, which is theU.S. national phase entry of PCT patent application no.PCT/EP2013/068669, which was filed on Sep. 10, 2013, which claims thebenefit of priority of U.S. provisional application no. 61/715,167,which was filed on Oct. 17, 2012, each of which are incorporated hereinin its entirety by reference.

FIELD

The present invention relates to a sensor system to measure a physicalquantity, a lithographic apparatus comprising the sensor system, and amethod of transferring a pattern from a patterning device onto a targetportion of a substrate using the sensor system.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In such a case, a patterning device, which isalternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Conventional lithographicapparatus include so-called steppers, in which each target portion isirradiated by exposing an entire pattern onto the target portion atonce, and so-called scanners, in which each target portion is irradiatedby scanning the pattern through a radiation beam in a given direction(the “scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In a lithographic apparatus, many sensor systems are used to measure allkinds of physical quantities. Examples of interesting quantities aredistance/position, time, speed, acceleration, force, lens aberration,etc, Some of these sensor systems use a detector that outputs aperiodically varying signal. Such a periodically varying signal may beobtained using a periodic structure, such as a grating. The periodicvarying signal may have, for instance, a sinusoidal shape.

SUMMARY

In the situation of multiple measurements at different spatiallocations, the measurement time may be reduced by using a paralleldetection arrangement with multiple detectors allowing measurements inparallel, e.g., at the same time, at different spatial locations. Whenusing such a parallel detection arrangement, it is advantageous, forexample, from a production point of view and a cost point of view toshare the same components such as power or signal components, wherepower components are used to provide energy to the detectors and thesignal components are usually used to manipulate the signal in a director indirect way.

However, although a parallel detection arrangement improves measurementspeed, it remains challenging for such a sensor system to maintain goodmeasurement reproducibility or even improve it when stricter demands mayrequire that. An obvious way of improving measurement reproducibility isto increase the measurement time, but in a lithographic apparatus thismay not be a viable option because of throughput demands.

It is desirable to provide, for example, a sensor system to measure aphysical quantity, wherein the sensor system has improved measurementreproducibility.

According to an embodiment of the invention, there is provided a sensorsystem to measure a physical quantity, the system including a paralleldetection arrangement with multiple detectors to allow measurements inparallel at different spatial locations, wherein the multiple detectorsshare at least one noise source, wherein the sensor system is configuredsuch that the multiple detectors each output a signal as a function ofthe physical quantity, and wherein the sensor system is configured suchthat at least one detector responds differently to noise originatingfrom the shared noise source than the other detectors.

According to an embodiment of the invention, there is provided a sensorsystem to measure a physical quantity in at least two directions, thesystem including a parallel detection arrangement with multipledetectors to allow measurements in parallel at different spatiallocations, wherein the multiple detectors share at least one noisesource, wherein each detector is configured to measure in one directionof the at least two directions at a time, wherein the sensor system isconfigured such that the multiple detectors each output a signal as afunction of the physical quantity, and wherein the sensor system isconfigured such that during a parallel measurement at least one detectoris measuring in a direction different from the other detectors at thesame time.

According to an embodiment of the invention, there is provided a controlsystem comprising a sensor system as described herein, at least oneactuator, and a control unit configured to provide a drive signal to theat least one actuator based on the output of the multiple detectors.

According to an embodiment of the invention, there is provided alithographic apparatus comprising a sensor system as described herein.

According to an embodiment of the invention, there is provided a patterntransferring method comprising:

measuring a position of a patterning device relative to a substratetable configured to hold a substrate using a sensor system as describedherein, the patterning device configured to impart a radiation beam witha pattern in its cross-section to form a patterned radiation beam;

aligning a target portion on a substrate held by the substrate tablewith the patterning device based on the measured position; and

projecting the patterned radiation beam onto the target portion of thesubstrate to transfer a pattern from the patterning device to thesubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2A depicts a part of the lithographic apparatus of FIG. 1 in moredetail and a detector of a sensor system according to an embodiment ofthe invention;

FIG. 2B depicts an input grating that can be used in the sensor systemof FIG. 2A;

FIG. 2C depicts a detection grating that can be used in the sensorsystem of FIG. 2A;

FIG. 2D depicts schematically a parallel detection arrangement usingmultiple detectors according to FIG. 2A;

FIG. 3 depicts a possible output of a straightforward parallel detectionarrangement;

FIG. 4 depicts a possible output of a parallel detection arrangementaccording to an embodiment of the invention; and

FIG. 5 depicts a possible output of a parallel detection arrangementaccording to an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation or EUV radiation).

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WTa or WTb constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore tables (and/or two or more patterning device tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure. As an example, the twosubstrate tables WTa and WTb in the example of FIG. 1 are anillustration of this. An embodiment of the invention disclosed hereincan be used in a stand-alone fashion, but in particular it can provideadditional functions in the pre-exposure measurement stage of eithersingle- or multi-stage apparatuses. In an embodiment, the lithographicapparatus may have a substrate table and a measurement table, whereinthe measurement table is not designed to hold a substrate (and is designto provide measurement functionality and optionally other functionality,such as cleaning).

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table) MT, andis patterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WTa/WTb can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WTa/WTb may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate tableWTa/WTb are kept essentially stationary, while an entire patternimparted to the radiation beam is projected onto a target portion C atone time (i.e. a single static exposure). The substrate table WTa/WTb isthen shifted in the X and/or Y direction so that a different targetportion C can be exposed. In step mode, the maximum size of the exposurefield limits the size of the target portion C imaged in a single staticexposure.

2. In scan mode, the support structure MT and the substrate tableWTa/WTb are scanned synchronously while a pattern imparted to theradiation beam is projected onto a target portion C (i.e. a singledynamic exposure). The velocity and direction of the substrate tableWTa/WTb relative to the support structure MT may be determined by the(de-)magnification and image reversal characteristics of the projectionsystem PS. In scan mode, the maximum size of the exposure field limitsthe width (in the non-scanning direction) of the target portion in asingle dynamic exposure, whereas the length of the scanning motiondetermines the height (in the scanning direction) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WTa/WTb is moved or scanned while a pattern imparted to theradiation beam is projected onto a target portion C. In this mode,generally a pulsed radiation source is employed and the programmablepatterning device is updated as required after each movement of thesubstrate table WTa/WTb or in between successive radiation pulses duringa scan. This mode of operation can be readily applied to masklesslithography that utilizes programmable patterning device, such as aprogrammable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Lithographic apparatus LA is of a so-called dual stage type which hastwo tables WTa and WTb and two stations, e.g., an exposure station and ameasurement station, between which the tables may be exchanged. Forexample, while a substrate on a substrate table is being exposed at theexposure station, another substrate can be loaded onto another substratetable at the measurement station or a measurement table may be locatedat the measurement station, so that various preparatory steps may becarried out. The preparatory steps may include mapping the surface ofthe substrate using a level sensor LS and measuring the position ofalignment markers using an alignment sensor AS. This enables asubstantial increase in the throughput of the apparatus. If the positionsensor IF is not capable of measuring the position of the table while itis at the measurement station as well as at the exposure station, asecond position sensor may be provided to enable the positions of thetable to be tracked at both stations. In a variation, for example, theapparatus may comprise a measurement table WTb and a substrate tableWTa. In this variation, while the substrate table WTa is at ameasurement station (where, for example, the substrate is unloaded andmeasurements do not necessarily occur at that station), the measurementtable WTb is located at the exposure station to enable measurements(e.g., measurements using the projection system).

The apparatus further includes a lithographic apparatus control unitLACU which controls all the movements and measurements of the variousactuators and sensors described. Control unit LACU also includes signalprocessing and data processing capacity to implement desiredcalculations relevant to the operation of the apparatus. In practice,control unit LACU will be realized as a system of many sub-units, eachhandling the real-time data acquisition, processing and control of asubsystem or component within the apparatus. For example, one processingsubsystem may be dedicated to servo control of the positioner PW.Separate units may even handle coarse and fine actuators, or differentaxes. Another unit might be dedicated to the readout of the positionsensor IF. Overall control of the apparatus may be controlled by acentral processing unit, communicating with these sub-systems processingunits, with operators and with other apparatuses involved in thelithographic manufacturing process.

FIG. 2A depicts in more detail a part of the lithographic apparatus ofFIG. 1. Shown schematically is the patterning device MA which issupported by the support structure MT and can be moved in one or moredirections by the first positioner PM (indicated schematically by dashedlines). Also schematically shown is the table WTa/WTb, which can bemoved by the second positioner PW (indicated by dashed lines). Theprojection system PS is also schematically depicted here, wherein onlyan upper and lower portion are schematically drawn.

FIG. 2A further depicts a detector of a sensor system which can be usedto measure aberration in the projection system and/or the position ofthe patterning device MA relative to the table WTa/WTb. The sensorsystem comprises a laser output LAS or any other suitable radiationsource output configured to provide a measurement beam MB. In anembodiment, the sensor system comprises a laser or any other suitableradiation source associated with the output LAS. The measurement beamfrom the output LAS is first spread by a diffuser DI and subsequentlyfocused by optical element L1 onto an input grating GR1, in this case apatterning device grating GR1 provided on or associated with thepatterning device MA.

The grating GR1 modulates the radiation from the laser LAS in a certaindirection to form a modulated measurement beam and the modulatedradiation is subsequently passed through the projection system PS. Theprojection system PS forms an image of the modulated measurement beamthat is projected onto a detection grating GR2 provided, for example, onthe table WTa/WTb. The interaction between the image from the projectionsystem PS and the detection grating GR2 provides a plurality ofoverlapping wavefronts which will interfere with each other. Theinterference pattern is detected by a camera CA, e.g. a CCD camera,positioned at a distance from the detection grating GR2. Aberrationspresent in the wavefronts and the relative position between the inputgrating GR1 and the detection grating GR2 will influence the resultinginterference patterns.

Usually, the patterning device and detection grating are steppedrelative to each other in a direction corresponding to the modulation ofthe modulated measurement beam and an image is captured by the camera ateach step. The intensity data obtained by the camera and representingthe overlapping and interfering copies of the wavefront is processed ina camera processing unit CP, where it may for instance be fitted toZernike polynomials to yield Zernike coefficients, each Zernikecoefficient providing information about a position or particularaberration in the modulation direction. For the position, the outcome isa periodically varying signal of which the period is determined by thepitch of the input and detection gratings.

In a similar way, information may be obtained about aberration and/orposition in a direction perpendicular to the above described modulationdirection. To allow this, the input grating GR1 may comprise two partsGR1 x and GR1 y, see FIG. 2B, wherein during a first measurement the GR1x part is used to modulate the measurement beam and during a secondmeasurement the GR1 y part is used to modulate the measurement beam.Because the lines of the two grating parts GR1 x and GR1 y areorthogonal to each other, the modulation directions associated with thegrating parts are also orthogonal to each other.

The detection grating GR2 may also comprise two parts corresponding tothe two grating parts GR1 x and GR1 y, but it is also possible to use asingle grating in the form of a checkerboard as shown in FIG. 2C, whichcan be used for both grating parts GR1 x and GR1 y. It will be apparentto a person skilled in the art of such sensor systems that there existmany grating variants that can be used to obtain aberration and/orposition information in two directions in a similar manner as describedin relationship to FIGS. 2A, 2B and 2C. These variants will notexplicitly be described here, but fall within the scope of an embodimentof the invention.

In order to quickly measure at different spatial locations, multipledetectors as depicted in FIG. 2A may be provided, hence, multiple inputgratings GR1 are provided, e.g., on the patterning device MA, andmultiple corresponding detection gratings GR2 are provided on, e.g., thetable WTa/WTb. By illuminating the gratings GR1 at substantially thesame or similar time, and capturing the images from the detectiongratings GR2 at the substantially same or similar time a paralleldetection arrangement is provided in which parallel measurements can betaken at the substantially same or similar time at different spatiallocations.

FIG. 2D schematically depicts the parallel detection arrangementmentioned above, wherein multiple detectors according to FIG. 2A, in theexample of FIG. 2D four detectors, share the same laser output LAS. Theradiation is split into four measurements beams by four diffusersDI1-DI4, which also spread the radiation. The measurement beams arefocused onto the input gratings GR1 on, e.g., the patterning device andsubsequently passed through the projection system to be projected ontothe detection gratings GR2. The images from the detection gratings aresimultaneously captured by the camera CA that is also shared by themultiple detectors.

The sensor system as depicted in FIGS. 2A and 2D can be used in acontrol system, which control system also comprises one or moreactuators to manipulate one or more components of the lithographicapparatus. In the embodiment of FIG. 2A, the shown first positioner PMhas an actuator to move the support structure MT including patterningdevice MA, the shown second positioner PW has an actuator to move thetable WT, and one or more actuators PSA is indicated to manipulate theposition and/or shape of one or more optical components, such as a lensand/or mirror.

The control system includes a control unit CU configured to providedrive signals to one or more different actuators on the basis of themeasured aberration and/or position. For instance, the drive signalprovided to the actuator PSA by the control unit is intended to reduceor minimize the aberration (and thus improve or optimize the performanceof the projection system) and/or the drive signal provided to the firstand/or second positioner may be intended to align the patterning deviceMA to the table WTa/WTb.

Illuminating the gratings GR1 at the same time can be done usingseparate outputs LAS and/or separate radiation sources. In anembodiment, a common radiation source and/or radiation output is used toilluminate all gratings GR1 at the substantially same or similar time.In the same manner, the images from the detection gratings GR2 can becaptured by individual cameras CA, but it is more cost effective to usea single camera CA capturing all images at once. Using a single cameraCA may also be advantageous from production point of view.

The straightforward implementation of a parallel detection arrangementis to provide multiple, as identical as possible, detectors next to eachother, so that the periodically varying signals as a function of therespective physical quantity are also substantially identical. This isshown as an example in FIG. 3, in which a sensor system including sevendetectors provides as many periodically varying signals. In FIG. 3, oneperiod of each of the seven periodically varying signals SI1-SI7 as afunction of position in one direction are shown, wherein eachperiodically varying signal is associated with a distinct spatiallocation.

In FIG. 3 it can easily be seen that correlated intensity noise in acommon camera and/or radiation source (e.g., a laser) results in anapparent position deviation or shift as measured by the sensor system atthe seven locations. As an example, seven data points DP1-DP7 aredepicted in FIG. 3, wherein each data point is associated with arespective periodically varying signal and indicates a measured positionin the case of the absence of noise. When, for instance, intensity noisefrom a shared noise source results in a simultaneous increase of theintensity of the signals SI1-S17, the data points DP1-DP7 shift to theright in FIG. 3 which is interpreted as a common shift in position. Asit is the measured position including noise that will be used to controlthe actuator, e.g., the first and/or second positioner PM, PW, themovable object, e.g., the patterning device MA, will not be alignedproperly with respect to another object, e.g., the table WTa/WTb, sothat errors may occur, such as overlay errors when transferring apattern to a substrate supported on the substrate table.

The correlated intensity noise may not only come from a shared noisesource such as the camera or radiation source, but may be caused by anypower or signal component that is shared by all detectors, e.g. a powersupply.

In an implementation of a parallel detection arrangement according to anembodiment of the invention, at least one of the detectors respondsdifferently to noise originating from a shared noise source than on oneor more other detectors, for instance because the periodically varyingsignal of at least one detector is different in phase and/or period fromthe periodically varying signal of one or more other detectors. As aresult thereof, the correlated intensity noise will have a differenteffect on the periodically varying signals depending on phase and/orperiod, which is not consistent with a pure shift in position. Thisallows for distinguishing between a real shift in position and theinfluence of noise.

In FIG. 4, seven signals SI1-SI7 are depicted originating from a sensorsystem according to an embodiment of the invention using a paralleldetection arrangement with seven detectors, e.g. seven detectors similarto the detector shown in FIG. 2A. The seven signals are comparable tothe seven signals of FIG. 3, but in the embodiment of FIG. 4 the secondand sixth detectors have a periodically varying signal SI2, SI6 with aphase difference of substantially 180 degrees relative to theperiodically varying signals SI1, SI3-SI5, SI7 of the other detectors.Hence, there exists a first sub-set of detectors SI1, SI3-SI5, SI7 and asecond sub-set of detectors SI2, SI6 having a periodically varyingsignal with a phase that is opposite to the phase of the periodicallyvarying signal of the first sub-set of detectors.

In case of a real position shift to the right, i.e. in case the datapoints DP1-DP7 move to the right in FIG. 4, the second and sixthdetectors will show a decrease in signal intensity, whereas the otherdetectors will show an increase in signal intensity. In the case ofcorrelated intensity noise, all detectors will show an increase insignal intensity, so that for the second and sixth detectors, theposition seems to have been shifted to the left, whereas the otherdetectors indicate that the position has shifted to the right in FIG. 4.Hence, there are conflicting indications in case of correlated noise.

This can advantageously be used in a control system when the conflictingindications can not easily be followed by the control unit and the oneor more actuators. In that case, the influence of the correlatedintensity noise is reduced although the reproducibility of theindividual detectors has not been improved. Hence, which one or moredetectors need to change phase relative to the one or more otherdetectors may be based on the (im)possibilities of the one or moreactuators. In an embodiment, the combination of detectors is used thatgives the best result.

In other words, the variation in drive signal to the actuator as aresult of a signal variation of the multiple detectors due to commonnoise is less than the variation in the drive signal to the actuator asa result of a signal variation due to a corresponding physical quantityvariation, in this case a corresponding position shift.

Shifting the phase of the second and sixth detectors by 180 degrees canbe done by adapting the gratings GR1. The applicable one or moregratings can be shifted half a period or the lines and spaces can beinterchanged. Alternatively, the detector gratings GR2 can be shiftedhalf a period.

FIG. 5 depicts seven signals SI1-SI7 of a sensor system according to afurther embodiment using a parallel detection arrangement with sevendetectors similar to the embodiment of FIG. 4. In this embodiment, thesecond, fourth and sixth detector have a periodically varying signalSI2, SI4, SI6 with a phase difference of substantially 90 degreesrelative to the periodically varying signals SI1, SI3, SI5, SI7 of theother detectors.

The embodiment of FIG. 5 employs the fact that a detector is mostsensitive to a change in the physical quantity, in this case position,in the ‘flanks’ of the periodically varying signal, and is lesssensitive at a maximum or minimum of the periodically varying signal.Due to the phase difference of 90 degrees, the second, fourth and sixthdetectors are most sensitive to a change in position when the otherdetectors are not and vice versa. Thus when obtaining multiple datapoints, the data points that are used to determine the position, i.e.the data points in the ‘flanks’ of the periodically varying signals,were not obtained at the same time, and thus the correlation thatoriginally existed between the noise at all detectors is no longerpresent and thus the influence of the noise will average out to acertain extent.

The same effect of breaking up the correlation between noise atdifferent detectors may be achieved by interchanging the gratingportions GP1 x and GP1 y of some of the detectors, so that during afirst measurement information is obtained by a first sub-set ofdetectors in a first direction and by a second sub-set of detectors in asecond direction, and that during a second measurement information isobtained by the first sub-set of detectors in the second direction andby the second sub-set of detectors in the first direction, wherein afterthe two measurements the information in each direction is combined. Dueto the fact that the information associated with one direction is notobtained at the same time, results in the influence of the noise beingaveraged out to a certain extent.

In an embodiment, there is provided a sensor system to measure aphysical quantity, the system including a parallel detection arrangementwith multiple detectors to allow measurements in parallel at differentspatial locations, wherein the multiple detectors share a noise source,wherein the sensor system is configured such that the multiple detectorseach output a signal as a function of the physical quantity, and whereinthe sensor system is configured such that at least one detector respondsdifferently to noise originating from the shared noise source than theone or more other detectors.

In an embodiment, the multiple detectors output a periodically varyingsignal as a function of the physical quantity, and wherein theperiodically varying signal from at least one detector differs in periodand/or phase from the one or more other detectors. In an embodiment, theat least one detector responds differently to noise originating from theshared noise source, due to the periodically varying signal of the atleast one detector differing in period and/or phase from the one or moreother detectors. In an embodiment, the periodically varying signal ofthe at least one detector differs in phase from the one or more otherdetectors, and wherein the difference in phase is substantially 180degrees. In an embodiment, the periodically varying signal of the atleast one detector differs in phase from the one or more otherdetectors, and wherein the difference in phase is substantially 90degrees. In an embodiment, the periodically varying signal of the atleast one detector differs in period from the one or more otherdetectors, and wherein the ratio between the periods is at least 2. Inan embodiment, each detector comprises: a radiation output to provide ameasurement beam; an input grating to modulate the measurement beam; adetection grating to create multiple overlapping and interfering copiesof a wavefront of the modulated measurement beam after it has passed anoptical system; and a camera arranged at a distance from the detectiongrating to capture an image of the overlapping and interfering copies ofthe wavefront, wherein the difference in period and/or phase between theperiodically varying signals of the detectors is caused by differencesin the input and/or detection gratings.

In an embodiment, there is provided a sensor system to measure aphysical quantity in at least two directions, the system including aparallel detection arrangement with multiple detectors to allowmeasurements in parallel at different spatial locations, wherein themultiple detectors share a noise source, wherein each detector isconfigured to measure in one direction of the at least two directions ata time, wherein the sensor system is configured such that the multipledetectors each output a signal as a function of the physical quantity,and wherein the sensor system is configured such that during a parallelmeasurement at least one detector is measuring in a direction differentfrom the one or more other detectors at the same time.

In an embodiment, the multiple detectors output a periodically varyingsignal as a function of the physical quantity. In an embodiment, eachdetector comprises: a radiation output to provide a measurement beam; aninput grating to modulate the measurement beam; a detection grating tocreate multiple overlapping and interfering copies of a wavefront of themodulated measurement beam after it has passed an optical system; and acamera arranged at a distance from the detection grating to capture animage of the overlapping and interfering copies of the wavefront,wherein the measurement direction is determined by the input and/ordetection grating. In an embodiment, the multiple detectors share thesame radiation output and/or share the same camera as the noise source.

In an embodiment, there is provided a control system comprising a sensorsystem as described herein, an actuator, and a control unit configuredto provide a drive signal to the actuator based on the output of themultiple detectors. In an embodiment, the multiple detectors and theactuator are configured such that a signal variation in the output ofthe multiple detectors due to noise originating from the noise sourcecannot be followed or cannot be followed completely by the actuator.

In an embodiment, there is provided a lithographic apparatus comprisinga sensor system as described herein. In an embodiment, there is provideda lithographic apparatus comprising: a sensor system as describedherein; and a pattern transfer system configured to provide a patternonto a substrate. In an embodiment, each detector of the sensor systemcomprises: a radiation output to provide a measurement beam; an inputgrating to modulate the measurement beam to form a modulated measurementbeam; a detection grating to create multiple overlapping and interferingcopies of a wavefront of the modulated measurement beam; and a cameraarranged at a distance from the detection grating to capture an image ofthe overlapping and interfering copies of the wavefront, wherein thelithographic apparatus further comprises: a support constructed tosupport a patterning device, the patterning device comprising the inputgratings of the multiple detectors; a substrate table constructed tohold a substrate, the substrate table comprising the detection gratings;and a projection system configured to project the modulated measurementbeams onto the respective detection gratings on the substrate table,wherein the physical quantity to be measured by the sensor system is theposition of the input gratings relative to the substrate table. In anembodiment, the lithographic apparatus comprises a first positioner toposition the support and/or a second positioner to position thesubstrate table, wherein the lithographic apparatus comprises a controlunit configured to provide drive signals to the first and/or secondpositioner based on an output of the multiple detectors.

In an embodiment, there is provided a pattern transferring methodcomprising: measuring a position of a patterning device relative to asubstrate table configured to hold a substrate using a sensor system asdescribed herein, the patterning device configured to impart a radiationbeam with a pattern in its cross-section to form a patterned radiationbeam; aligning a target portion on a substrate held by the substratetable with the patterning device based on the measured position; andprojecting the patterned radiation beam onto the target portion of thesubstrate to transfer a pattern from the patterning device to thesubstrate. In an embodiment, the patterning device comprises a gratingper detector and wherein the position of the patterning device ismeasured by illuminating the grating of the patterning device with ameasurement beam to form a modulated measurement beam, projecting themodulated measurement beam onto a detection grating provided on thesubstrate table to create multiple overlapping and interfering copies ofa wavefront of the modulated measurement beam, capturing an image of theoverlapping and interfering copies of the wavefront, and calculating theposition of the patterning device relative to the substrate table fromthe captured image.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1.-7. (canceled)
 8. A sensor system to measure a physical quantity in atleast two directions, the system including a parallel detectionarrangement with multiple detectors to allow measurements in parallel atdifferent spatial locations, wherein the multiple detectors share anoise source, wherein each detector of the multiple detectors isconfigured to measure in one direction of the at least two directions ata time, wherein the sensor system is configured such that the multipledetectors each output a signal as a function of the physical quantity,and wherein the sensor system is configured such that during a parallelmeasurement at least one detector of the multiple detectors is measuringin a direction different from the one or more other detectors of themultiple detectors at the same time.
 9. The sensor system according toclaim 8, wherein the multiple detectors output a periodically varyingsignal as a function of the physical quantity.
 10. The sensor systemaccording to claim 8, wherein each detector of the multiple detectorscomprises: a detection grating to create multiple overlapping andinterfering copies of a wavefront of a measurement beam modulated by aninput grating after it has passed an optical system; and a cameraarranged at a distance from the detection grating to capture an image ofthe overlapping and interfering copies of the wavefront, wherein themeasurement direction is determined by the input and/or detectiongrating.
 11. The sensor system according to claim 8, wherein themultiple detectors share a same radiation output and/or share a samecamera as the noise source.
 12. A control system comprising the sensorsystem according to claim 8, an actuator, and a control unit configuredto provide a drive signal to the actuator based on the output of themultiple detectors.
 13. The control system according to claim 12,wherein the multiple detectors and the actuator are configured such thata signal variation in the output of the multiple detectors due to noiseoriginating from the noise source cannot be followed or cannot befollowed completely by the actuator.
 14. A lithographic apparatuscomprising: the sensor system according to claim 8; and a patterntransfer system configured to provide a pattern onto a substrate. 15.The lithographic apparatus according to claim 14, wherein each detectorof the multiple detectors of the sensor system comprises: a detectiongrating to create multiple overlapping and interfering copies of awavefront of a measurement beam modulated by an input grating; and acamera arranged at a distance from the detection grating to capture animage of the overlapping and interfering copies of the wavefront,wherein the pattern transfer system further comprises: a supportconstructed to support a patterning device, the patterning devicecomprising input gratings of the multiple detectors; a substrate tableconstructed to hold a substrate, the substrate table comprising thedetection gratings; and a projection system configured to project themodulated measurement beams onto the respective detection gratings onthe substrate table, wherein the physical quantity to be measured by thesensor system is the position of the input gratings relative to thesubstrate table.
 16. The lithographic apparatus according to claim 15,comprising a first positioner to position the support and/or a secondpositioner to position the substrate table, wherein the lithographicapparatus comprises a control unit configured to provide drive signalsto the first and/or second positioner based on an output of the multipledetectors.
 17. A pattern transferring method comprising: measuring aposition of a patterning device relative to a substrate table configuredto hold a substrate using a sensor system according to claim 1, thepatterning device configured to impart a radiation beam with a patternin its cross-section to form a patterned radiation beam and the sensorsystem including a parallel detection arrangement with multipledetectors to allow measurements in parallel at different spatiallocations, wherein the multiple detectors share a noise source, whereineach detector of the multiple detectors is configured to measure in onedirection of the at least two directions at a time, wherein the sensorsystem is configured such that the multiple detectors each output asignal as a function of a physical quantity, and wherein the sensorsystem is configured such that during a parallel measurement at leastone detector of the multiple detectors is measuring in a directiondifferent from the one or more other detectors of the multiple detectorsat the same time; aligning a target portion on a substrate held by thesubstrate table with the patterning device based on the measuredposition; and projecting the patterned radiation beam onto the targetportion of the substrate to transfer a pattern from the patterningdevice to the substrate.
 18. The pattern transferring method accordingto claim 17, wherein the patterning device comprises a grating perdetector of the multiple detectors and wherein the position of thepatterning device is measured by illuminating the grating of thepatterning device with a measurement beam to form a modulatedmeasurement beam, projecting the modulated measurement beam onto adetection grating provided on the substrate table to create multipleoverlapping and interfering copies of a wavefront of the modulatedmeasurement beam, capturing an image of the overlapping and interferingcopies of the wavefront, and calculating the position of the patterningdevice relative to the substrate table from the captured image.
 19. Thepattern transferring method according to claim 17, wherein at least twodetectors of the one or more other detectors respond substantially thesame to noise originating from the shared noise source.
 20. The sensorsystem according to claim 8, wherein at least two detectors of the oneor more other detectors respond substantially the same to noiseoriginating from the shared noise source.
 21. The sensor systemaccording to claim 9, wherein the periodically varying signal from theat least one detector differs in period and/or phase from the one ormore other detectors.
 22. The sensor system according to claim 21,wherein the at least one detector responds differently to noiseoriginating from the shared noise source, due to the periodicallyvarying signal of the at least one detector differing in period and/orphase from the one or more other detectors.
 23. The sensor systemaccording to claim 21, wherein the periodically varying signal of the atleast one detector differs in phase from the one or more otherdetectors, and wherein the difference in phase is substantially 180degrees.
 24. The sensor system according to claim 21, wherein theperiodically varying signal of the at least one detector differs inphase from the one or more other detectors, and wherein the differencein phase is substantially 90 degrees.
 25. The sensor system according toclaim 21, wherein the periodically varying signal of the at least onedetector differs in period from the one or more other detectors, andwherein the ratio between the periods is at least
 2. 26. A methodcomprising: measuring radiation by multiple detectors to allowmeasurements in parallel at different spatial locations, wherein themultiple detectors share a noise source, wherein each detector of themultiple detectors is configured to measure in one direction of at leasttwo directions at a time, wherein the multiple detectors each output asignal as a function of a physical quantity, and wherein during aparallel measurement, at least one detector of the multiple detectors ismeasuring in a direction different from the one or more other detectorsof the multiple detectors at the same time; and deriving a value of thephysical quantity based on the output of the multiple detectors.
 27. Themethod according to claim 26, wherein the multiple detectors output aperiodically varying signal as a function of the physical quantity, andwherein the periodically varying signal from the at least one detectordiffers in period and/or phase from the one or more other detectors.